Feedback circuit and radio receiver



Jan. 16, 1940. PRATT 2,187,402

I FEEDBACK CIRCUIT AND RADIO RECEIVER Filed Jul 21,1958

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WQMW Patented Jan. 16, 1940 east rrlfiE 2,187,402 FEEDBACK cinema? AND RADIO RECEIVER Rodney FQPratt, Gi'aliamsville, N. Y., assignor of one-half to Manuel Dittenheimer, Ellenville,

Application July 21, 1938, Sa aam. 220,547 7 3 c aims. (01. na -171) My invention relates to improvements in radio receiving and amplifying systems and the like and has for its objects increasingthe amplification. and efiectiveness of the vacuum tubes employed together .with improvement in tone quality obtained and the reduction of electric current required resulting in unusual economy of operation. i i

In the drawing, Figure l, is shown as three tube radio receivingcircuit suitable for the ap plication of the principles of my invention wherein the circuit elements selected are of un- Referring to the drawing, Tl, T2 and T3 are vacuum tubes, Tl here. being employed as a de- 2 tector, as an intensifier, and T3 as an amplioroutput tube, said tubes being operated in cascade. Ll is a tuning inductance or coil which may be provided with tape connecting with a suitable tap switch Si and having connected -3 across cell a variable tuning condenser Cl. Antenna and ground connections are provided through fixed condensers Cl! and G l, respectively,

(32 being of very small capacity. A grid condenser C2; is connected from the tuned circuit Li, Cl to the grid of tube Ti and a leak resistance R3 is provided. ,The plate of tube TI ,is connected through a reaction coil L2 placed in suitable inductive relation to. coil LI and a resister Rl to a positive. plate current. supply as the battery B2. A suitable radiofrequency bypass condenser is provided at CB and a very small capacity condenserrnay be used at C5. The plate or tube Tl is also connected through a radioffrequency choke coil L3 anda direct curmo rent blocking condenser C7 to the grid of tube T2, 2. grid resistance for tube Ttbeing indicated at B4; A by-pass condenser Cd is connected from choke L3 to the tube filament circuit and serves to by-pass or attenuate radio frequency currents that otherwise may pass through choke L3 into the audio amplifier tubes. The plate of tube T2 is connectedfthrcugh a plateresistance R2 to the plate battery Biiand also is connected through a blocking condenser C8 to thegrid of tube T3 for which grid resistance Rbis provided. Condenser Cll of small capacity is usedas a bypass across the plate of tubeTil. Theplate of tube T3 is, connected through asuitable output device or loudspeaker (not shown) to the plate battery B2 and a condenser CH] of suitable ca pacity may be connectedacross the output circuit. A suitable battery Bl or other means may be used for heating the filaments of tubes Ti,

and TS.

Particular features of my invention that distinguish it from what is known in the art appear in the use of circuit elements of unusual Values which, in conjunction with the coil design and coupling and the circuits employed, produce results differing in marked degree from those ordinarilyexpected. All tubes operated with below normal or rated filament volt age, or relatively low operating temperature, plate resistor Rl is of unusual high value being variable with a maximum value of ten rnegohnis, plate resistor R2 is of a maximumvalue of three megohms, grid resistors R3 and R5 are of relatively high value being of the order of two to ten inegohms and resistor R4 is of high. value of the order of. ten megohnis. These high plate resistor values are used in conjunction with a platefsupply voltage of relatively low value such as eighteen to forty-five volts, Operation and performance of tube T3 may be substantially conventional; unusual pericrmance being obtained particularly in tube T2 and in part in tube Tl, or both together.

Capacity CM represents the grid to plate ca-' pacityof tube Tl, which.v capacity may be increased by a small added condenser if desired. The grid to cathode and plate to cathode capacities of tube Tl are not separately indicated since they are in parallel with condensers (ll- 33 and C5 respectively. Mutual inductance between cells Li and L2 is represented by the negative mutual inductance; The plate resistance of tube Tl is represented by Rpl.

By reference to the equivalent diagrammatic circuit of Figure 2, important features of my invention and the marked difference with conventional practice may be explained. Conventional. circuits such as regenerative, super-regenerative and oscillating, as used in radio receiving sets, ,supcr-heterodynes, transmitters and. the like ernploying vacuum tubes, may all be analyzed as coupled circuits in which well known principles and conditions are commonly employed: in my invention, however, such widely different condi- ,tions are employed as to result in an entirely stood that periormance of such circuits depends upon resistance and coupling values as well as the natural frequency response of the individual inductance and capacity circuits. Conventional practice employing tubes operated under normal conditions where the plate resistance Rpl is relatively low requires that relatively small coupling be employed between coils LI and L2v in order that not too much damping effect be introduced into the main tuning circuit LlC.|; that is, the damping effect of low plate resistance Rpl or other parallel resistance of the plate circuit LZC--C6 must be kept ata satisfactory low value in the tuned circuit LICI by using relatively loose coupling if circuit operationis to be satisfactory. But, upon considering the coupled circuits with regard to their individual frequencies of response, particularly the tuned circuit formed by Ll-CI and that formed by Ll-Cl (parallel connected) in series with L2 and Cl I, these two circuits may have rather close natural frequencies of response when Cl is relatively small, and the coupled circuit so formed will have two frequencies of response relatively close together when coupling between Li and L2 is small. By employing close coupling between coils LI and L2 and keeping parallel circuit resistances relatively high in value, I avoid the damping effects and coupled circuit effects that would otherwise result in broad tuning and unsatisfactory performance.

It may be seen then that my approach toward obtaining satisfactory circuit performance is completely different from that employed in conventional practice. My plan is to start with a circuit designed to have suitable circuit characteristics'avoiding undesired coupled circuit effects and keep out damping effects by keeping tube and other parallel circuit resistances relatively high, while conventional practice is to start with tube or other parallel circuit resistances of relatively low or, normal values and keep coupling effects sufiiciently small to avoid excessive damping effects. In operation, my arrangement gives unusual and unexepected performance with the reasons for said performance being explained or suggested above. Coupled circuits, it will be understood, are quite difficult to explain and analyze completely, but some of the differences between my practice and conventional practice are believed to be clearly indicated. Practical tests have shown that 'my circuit may be operated, if desired, as a super-regenerative receiver without showing the broadness of tuning commonly associated with reception with this type of receiver. Also, the hiss and background whistle commonly associated with the super-regenerative type of receiver are greatly reduced in intensity or eliminated by the use of. my circuit arrangements.

It will be understood, of course, that I do not restrict the principles herein employed and described to any one particular type of radio receiver, transmitter or the like. In the elimination of hiss and other undesirable features, my circuit features may be employed to advantage in oscillators, super-heterodyne receivers and elsewhere.

Also, it will be understood that I do not limit the use of my principles to any particular type of tube or method of supplying operating voltages for same. Vacuum tube characteristics have been extensively investigated for what has been considered normal operating ranges, but comparatively little data appears to be available on the tube characteristics where the operating currents are extremely small. It is known, however, that the alternating current plate resistance (Rpl) of a tube increases as the applied plate potential is decreased. In some triodes, the A. C. plate resistance (R171) may be approximately one-quarter of the computed direct current resistance at normal operating conditions; and, by this approximation, since I have operated tubes successfully with a plate current of one micro-ampere or less, the indicated A. C. plate resistance (R2 1) may exceed one megohm though the same tube may have a rated A. C. plate resistance (RM) of about 10,000 ohms under normal operating conditions. It would seem, then, that successful operation of a tube with such extremely small plate current definitely proves a new and unexpected class of performance. In the amplification or mu of a tube is materially affected by this operation with extremely small plate current, it may be shown that the amplification obtainable from the circuit in which the tube operates is not dependent upon the mu of the tube, provided that mu is greater than unity. Regenerative circuits, or feed-back circuits, have been extensively investigated and it is known that tube circuits may provide very great amplification or a tube will generate sustained oscillations when the feed-back voltage approaches or exceeds the input voltage. Use of a tube having a high A. C. plate resistance (Rpl) at normal operating potentials, such as a tetrode or other high mu tube, seemingly would not permit the use of satisfactory circuit coupling values because of the high amplification or mu of the tube unless operating conditions of said tube be materially changed from the normal.

In practical tests and use of radio receivers employing the principles of my invention, a remarkable freedom from the obstacles or impediment commonly known in the radio field as interference is noticed. This freedom from interference may be difficult to explain, but it is present and seems due in large part to the coupled circuit conditions explained hereinbefore. Where coupled circuit conditions are such as here employed, giving response only to such radio frequency signals as are desired with little chance for response to additional frequencies or parasitics, it may be understood that radio receivers employing my principles will be relatively free from noise resulting from interference or shock excitation. It is known that additional response frequencies in radio oscillators and transmitters give rise to parasitics and in receivers it may be believed that analogous conditions give rise to noise or other interference.

To sum up, then, features up my invention: I employ an amplifying tube or tubes operated under special conditions giving unusual and unfamiliar performance; I eliminate or greatly reduce the damping effects on inductance and capacity circuits associated with said tube or tubes by tube plate circuit resistance and other circuit resistances at exceptionally high value relative to normal or usual values; and I employ a coupled circuit providing controlled feed-back, regeneration or intensification, and eliminate or reduce in said coupled circuit undesirable coupled circuit effects by suitable coupling and circuit element, values as explained above.

Having thus described my invention, what I claim is:

1. An amplifying tube and associated circuit comprising a tunable grid circuit having inductance and capacity and a plate circuit having relatively similar inductance and capacity closely coupled to said grid circuit, means for reducing the damping effect to negligible value of the tube plate circuit resistance upon said closely coupled grid andplate circuits comprising the operation of said amplifying tube with a relatively high external plate circuit resistance of the order of a megohm.

2. An amplifying tube and associated circuit comprising a tunable grid circuit having inductance and capacity and a plate circuit having relatively similar inductance and capacity closely coupled to said grid circuit, means for reducing the damping effect to negligible value of the tube plate circuit resistance upon said closely coupled grid and plate circuits comprising the operation of said amplifying tube with a relatively high external plate circuit resistance of the order of a megohm and means for increasing the internal plate resistance of said tube by operation of said tube cathode at a relatively low temperature.

3. An amplifying tube and associated circuit comprising a tunable grid circuit having inductance and capacity and a plate circuit having relatively similar inductance and capacity closely coupled to said grid circuit, means for reducing the damping effect to negligible value of the tube plate circuit resistance upon said closely coupled grid and plate circuits comprising the operation of said amplifying tube with a relatively high external plate circuit resistance of the order of a megohm and a capacity in parallel with the internal plate resistance of said tube having a reactance value greatly less than the plate resistance of said tube.

RODNEY F. PRATT. 

